Disclaimer: I'm not a doctor, so take this with for little it may be worth:

I can tell you what not to do. My urologist gave me 20 units for my first dosage in the clinic. It resulted in a priapism (staying rock hard for 4 hours+) needing a ER visit to drain blood and take it down.

I changed urologists and the new one recommended trying Bimix, starting with 7 units. Solid results, staying hard as 30 years ago for 2 hours. I'm sure Trimix would have been fine at the smaller dosage, but I was a bit paranoid.

The commendation given to me was to vary the dosage by a max of 2 units to see what works best.

Best suggestion: Ask a urologist!

I would suppose that the answer would depend on the formulation of the trimix that you are using.  They are not all the same.  I used to use trimix with good results about 5-years ago before my prostatectomy.  I used to inject it with an insulin syringe (Max capacity 1 ml or 100 units) usually using around 10 to 15 units.  For fresh trimix, usually 10 units worked fine, but as the trimix neared its expiration date I would usually up the dosage to about 15 or so units,  Unfortunately for me, after my prostatectomy, neither trimix nor bimix worked and both would cause a lot of pain.    

Every body is different.  I would recommend discussing with your urologist.    

Start at 10 units and increase or decrease by 5 units til you reach your target. Franktalk.org has lots of info for you to read and ask questions